Spacer plate in front of leaf spring hanger

The part you pulled has torch-cut slots. I doubt a factory part would be so crudely made. Plus, are they going to take the time on an assembly line to check that after the rear axle is in place? I don't think so.

I would speculate that it was added sometime in your car's previous life. Was it because of an accident? Might be something an alignment shop would do, or by a racer as others have mentioned.